Near field communication (“NFC”) devices are capable of communicating when they are placed in close proximity to each other, and may be used for transactions such as payment transactions. Each of the NFC communication devices includes an antenna and related circuitry such as a matching circuit. A first NFC communication device generates a wireless carrier signal at a suitable frequency such as 13.56 MHz and transmits that signal over its antenna. When the antenna of a second NFC communication device is placed in close proximity to the antenna of the first NFC communication device, the two devices become inductively coupled, such that energy is coupled between the two devices through a shared magnetic field.
When the two NFC communication devices are inductively coupled, either of the NFC communication devices may modify the wireless carrier signal. The first NFC communication device may modify the wireless carrier signal prior to transmission. These modifications change the amplitude and/or frequency of the wireless carrier signal in order to encode data that is transmitted to the second NFC communication device. The second NFC communication device also encodes data that is transmitted to the first NFC communication device. Because the second NFC communication device is not transmitting the wireless carrier signal, it cannot modify the signal prior to transmission. Instead, it changes a load applied to its antenna, thus modifying the magnetic field between the two NFC devices. This results in changes to the wireless carrier signal, which the second NFC communication device uses to encode data that is transmitted to the first NFC communication device.
The magnetic field between the NFC communication devices also depends on the relative position and physical packaging of the NFC communication devices. The magnetic field changes based on how close the devices are to each other as well as the orientation of the devices. One or both of the devices may be packaged within metals or non-conductive materials that impact that magnetic field. In some cases, the relative position and packaging may combine to impact the magnetic field in a complex manner, for example, when two devices are placed in close proximity to each other and the packaging of one of the devices includes ferrous materials. Because the relative position between the NFC devices is typically under human control, as in a payment transaction, the characteristics of the wireless carrier signal that is coupled between the NFC communication devices may be unpredictable or may change while the devices are communicating.